1. Field of the Invention
The present invention relates to a liquid crystal display panel to be used as a light bulb in a liquid crystal projector or the like (hereinafter simply referred to as “liquid crystal display panel”), and more specifically, relates to a structure of a black matrix that optically intercepts or blocks those portions other than pixel openings in the liquid crystal display panel and a method of fabricating such a black matrix.
2. Description of the Related Art
Generally, in a liquid crystal display panel, intense projected light is entered from the side of an opposite substrate that is disposed so as to confront a driving substrate (TFT (Thin Film Transistor) array substrate) with a liquid crystal phase being an electro-optic substance interposed therebetween. Such an opposite substrate may be often called a counter substrate or an opposing substrate.
If this intense projected light enters channel forming regions comprising a-Si (amorphous silicon) films or p-Si (polysilicon) films of TFTs disposed on the driving substrate, photoelectric current is generated in those regions due to the photoelectric transfer effect to deteriorate transistor characteristics of the TFTs. Therefore, in order to suppress this phenomenon, light-shielding films arranged in a matrix form, called a black matrix, are generally formed on the opposite substrate at positions confronting the respective TFTs.
In general, on the surface of the opposite substrate where the black matrix is formed, an ITO film serving as a transparent electrode for driving liquid crystals is formed, and further, an orientation film in the form of a polyimide thin film for controlling orientation of liquid crystal molecules is formed on the ITO (Indium Tin Oxide) film. This orientation film is subjected to a rubbing treatment wherein the surface of the film is rubbed by a roll made of nylon or the like to form stripes thereon.
The opposite substrate having been subjected to the rubbing treatment is joined with the driving substrate, and liquid crystals are poured in between the substrates that are then sealed, thereby to fabricate a liquid crystal display panel.
Recently, following the increased number of pixels in liquid crystal projectors, the size of openings of a black matrix formed on an opposite substrate used in a liquid crystal projector, an interval between the openings, a cell gap between the opposite substrate and a driving substrate, and so on, have been reduced.
For example, practically, the size of each opening of a black matrix is set to 14 μm, an interval between the openings (line width of black matrix) is set to 4 μm, a cell gap between a driving substrate and an opposite substrate is set to 3 to 4 μm.
Therefore, there has been a problem, wherein a malfunction of a liquid crystal display panel is caused by the fact that, due to intense projected light entering the liquid crystal display panel, stray light is generated in liquid crystal cells, the temperature of the liquid crystal display panel increases, and so on.
In view of this, JP-A-9-211439, for example, discloses that a high reflectance layer is first provided on a glass substrate forming an opposite substrate and then a low reflectance layer made of black resin or Cr oxide is provided thereon, thereby to form a black matrix on the glass substrate.
With this structure, a malfunction caused by stray light generated in liquid crystal cells is prevented by the low reflectance layer, while incident light applied to the black matrix is effectively reflected by the high reflectance layer so as to prevent the increase in temperature of the liquid crystal display panel itself.
However, following the increased number of pixels in a liquid crystal display panel, there has been raised a problem, wherein disturbance occurs in orientation of liquid crystal molecules in a liquid crystal display pane, thereby to cause unevenness or nonuniformity in a projected image.
In addition, upon attempting to fabricate a liquid crystal display panel according to the conventional technique, the present inventors have found that, in case a high reflectance film and a low reflectance film are made of different materials, when patterning the films by etching, it is difficult to control a pattern section due to different etching characteristics of the films caused by the different materials thereof.
Following the increased number of pixels in the liquid crystal display panel, the stability of the section shape of the black matrix pattern has been strictly required. Accordingly, the difficulty in etching control has been further increased.
In view of this, the present inventors have made researches about a cause that generates unevenness in the liquid crystal display panel. The result thereof will be briefly explained with reference to FIG. 3.
FIG. 3 is an exemplary sectional view of an opposite substrate according to the prior art.
The conventional opposite substrate 3 comprises a transparent substrate 10, a black matrix 25, a transparent conductive film 30 and an orientation film 40. The surface of the orientation film 40 includes a portion 41 formed over the transparent substrate 10, a portion 42 formed over the black matrix 25, and a portion 43 formed over a shoulder of the black matrix 25.
In the conventional opposite substrate 3, on the transparent substrate 10 (the side confronting a non-shown driving substrate), the black matrix 25 having a thickness of about 0.1 to 0.12 μm is formed, and thereon, the transparent conductive film 30 made of ITO or the like as a main component is formed, and thereon, the orientation film 40 made of polyimide or the like as a main component is formed.
Since the black matrix 25 has a pattern, the transparent conductive film 30 and the orientation film 40 also follow the pattern of the black matrix 25, so that steps are generated.
Therefore, on the surface of the orientation film 40, there appear the portion 41 formed over the transparent substrate 10, the portion 42 formed over the black matrix 25, and the portion 43 formed over the shoulder of the black matrix 25.
After the formation of the orientation film 40, a rubbing treatment is applied to the orientation film 40 to rub the surface thereof using a roll made of nylon or the like, thereby to form stripes thereon. In this event, the states of the rubbing treatment on the surface of the orientation film 40 differ among the portion 41 formed over the transparent substrate 10, the portion 42 formed over the black matrix 25, and the portion 43 formed over the shoulder of the black matrix 25. Particularly, in the portion 43 formed over the shoulder of the black matrix 25, unevenness of liquid crystal orientation is liable to occur due to nonuniformity of the rubbing treatment.
Accordingly, it has been found out that the nonuniformity of the rubbing treatment causes the disturbance of the liquid crystal molecule orientation in the liquid crystal display panel, resulting in the unevenness of the projected image.